A sintered bearing is used under a state in which inner pores are impregnated with a lubricating oil, and along with rotation of the sintered bearing relative to a shaft to be supported, the lubricating oil impregnating an inner portion exudes to a sliding part between the sintered bearing and the shaft, to thereby form an oil film through which the shaft is rotatably supported. Such a sintered bearing has been used as a bearing configured to rotatably support a small motor such as an HDD, an LBP polygon scanner motor, or a fan motor due to the high rotation accuracy and quietness thereof.
As the electronic device described above becomes thinner, the axial dimension of the sintered bearing is decreasing more and more. In the sintered bearing having such a small axial dimension, bearing stiffness (in particular, bearing stiffness in a radial direction) is required to be ensured. In particular, in the case where the sintered bearing is incorporated into a mobile telephone, a tablet terminal, or the like, high bearing stiffness is also required in the sintered bearing because those devices are used in various postures. However, the sintered bearing has an infinite number of pores opened in a bearing surface, and hence there is risk in that the oil film in the sliding part is released from openings of the bearing surface to an inside of the sintered bearing, that is, so-called dynamic pressure absence occurs, with the result that desired bearing stiffness may not be obtained. Such a situation can be avoided by subjecting the bearing surface to pore-sealing treatment by shot blasting, rotation sizing, or the like after forming the sintered bearing. However, in this case, manufacturing cost increases owing to an increase in man-hour.
In view of the above, for example, in Patent Literature 1, metal powder serving as a material for a sintered bearing is refined (the maximum particle diameter of tin powder is set to 25 μm or less, and the maximum particle diameters of copper powder and SUS powder are set to 50 μm or less), to thereby form fine surface openings in a bearing surface owing to the melting of the tin powder. Thus, the dynamic pressure absence is prevented.